The present invention relates to communication systems, and more particularly to free space laser communication systems.
Free space, point-to-point communication systems are used extensively in the communications field. A network of point-to-point microwave systems can carry messages across the country as part of the public switched telephone network. Despite strong competition from fiber optic based communications systems, microwave or other free space systems are often justified for shorter routes, when right-of-way for a cable system is not available, or when the high communications capacity of a fiber optic system is not needed.
Laser communication systems in particular have become increasingly popular to provide a free space communications link between two locations. Laser systems do not require extensive frequency coordination as do microwave systems in common frequency bands. Moreover, free space laser communication systems often are less expensive to install than either a conventional copper cable system or a fiber optics based system because physical installation of a cable or a fiber is unnecessary. For example, a laser communication system may have application between two corporate locations in a campus environment. Laser communication terminals may be positioned on building rooftops or adjacent windows and aligned to operate between buildings. Moreover, general progress in society is accompanied by increases in the amount of available information and, consequently, increased need for broader bandwidth communication systems. Accordingly, the demand for free space laser communications links is increasing. Such links can be used for communicating a variety of data forms including voice, video, and text.
Free space laser communication systems are considered stationary laser sources for governmental regulatory purposes. They must, therefore, comply with regulatory limits established to protect accidental observers. Exposure to high power laser beams may harm accidental observers. The harm by lasers used in free space communication can be compounded because (1) exposure to some laser wavelengths does not cause any pain that might otherwise warn one of exposure to the laser, and (2) some laser wavelengths are not visible.
Accordingly, standards have been put in place that establish safe limits for the power that may be transmitted by a stationary laser source, such as a laser communication terminal. These permissible power limits affect the communication system""s signal-to-noise ratio, operational bit rate, and/or useful distance coverage. To avoid deleterious effects on data communication while protecting accidental observers, U.S. Pat. No. 5,229,593 to Cato discloses an approach that runs the laser power of a free space laser communication system at regulatory established safe levels when exposure or misalignment is determined, otherwise the approach runs laser power at above the safe levels.
The approach disclosed by Cato uses a laser transmitter, laser receiver, and a controller to control the output laser power. In the disclosed approach, the received laser signal must contain a confirmation signal that a receiver down stream has received the transmitted laser signal if output laser power is to be maintained at above safe levels. The disclosed approach, however, suffers from certain disadvantages including not addressing atmospheric effects that might reduce received laser power as opposed to an accidental observer partially blocking the laser path and causing a reduction in the received laser power. The disclosed approach, moreover, does not address needs, implementations, or applications for one way laser communication since the disclosed approach requires a return laser signal carrying confirmation information.
The present invention presents an approach for free space laser communication that determines whether a reduction in received laser power is due to blockage or whether the reduction is due to atmospheric effects. The inventive approach allows for one-way and two-way laser communication. The inventive approach also allows for controlling the laser communication without suffering from line of sight or atmospheric effects.
The present invention achieves the above mentioned advantages by using a communication terminal that at least has (1) a laser transmitter that transmits a laser output beam; (2) a receiver that receives (by a communication channel that is not free space laser based) information about the power of the output laser beam measured at another communication terminal; and (3) a controller that controls the power of the output laser beam based on whether a reduction in received laser power is due to atmospheric effects or blockage. The receiver may be implemented as one way communication, or may be implemented as a transceiver allowing two way communication; the receiver/transceiver may communicate using broadcasting, telephone, limited bandwidth fiber communication, or any other non-free space laser communication approaches.
The present invention also achieves the above mentioned advantages by a related approach using a communication terminal that at least has (1) a laser transmitter that transmits a laser output beam; (2) first receiver that receives an input laser beam; (3) a second receiver that receives (by a communication channel that is not free space laser based) information about the power of the output laser beam measured at another communication terminal; and (4) a controller that controls the power of the output laser beam based on whether a reduction in received laser power is due to atmospheric effects or blockage. The second receiver may be implemented as one way communication, or may be implemented as a transceiver allowing two way communication; the receiver/transceiver may communicate using broadcasting, telephone, limited bandwidth fiber communication, or any other non-free space laser communication approaches.
The controller of a communication terminal according to the present invention uses information about the power of the transmitted laser beam that is received at a distant terminal, or information about the power of the laser beam input to the communication terminal, or both, to determine whether reduction in power of the transmitted laser received at some distance is due to atmospheric effects or blockage. The controller determines the nature of a reduction in the power of a laser beam using inherent differences in the amount and speed of a reduction in a laser power due to atmospheric effects and blockage. Generally, atmospheric effects occur at a time scale of the order of 10 milliseconds whereas blockage occurs at a time scale of the order of 100 milliseconds. The controller increases power of the output laser beam to above levels deemed safe to an accidental observer if it determines that a reduction in received laser power is due to atmospheric effects. On the other hand, the controller reduces power of the output laser beam to levels deemed safe to an accidental observer if it determines that the reduction in received laser power is due to blockage.